The invention relates to a diffusor pump, and more particularly to a diffusor blade for a diffusor pump.
A normal diffusor pump includes diffusors with a plurality of blades forcing a liquid to exhibit a flow straightening. A liquefied natural gas immersed pump (a LNG immersed pump) is one of typical diffusor pumps. The liquefied natural gas immersed pump also includes the diffusors, each of which has a side face provided with a plurality of blades.
The structure of a normal LNG immersed diffusor pump will be described with reference to FIG. 1. The LNG immersed diffusor pump comprises a motor section and a pump section. The pump section of the LNG immersed diffusor pump comprises a plurality of stages, each of which includes an impeller 10 and a diffusor 12. The diffusor 12 is provided at its side portion with a plurality of blades 14. A liquid flows through the impeller 10 by which a rise of pressure of the liquid appears. After that, the liquid flows on the side portion of the diffusor 12 with the blades 14 and then transmitted to a next stage.
The structure of the conventional diffusor 12 with the blades 14 involved in the LNG immersed diffusor pump will subsequently be described with reference to FIG. 2. The diffusor 12 comprises a cylindrical-shaped body. The cylindrical-shaped diffusor 12 has a side portion which is provided with a plurality of the diffusor blades 14. The plural diffusor blades 14 are so arranged as to be in parallel to each other at a predetermined interval. Each of the diffusor blades 14 is further so arranged as to have a longitudinal direction along a desired flow direction of the liquid, because the longitudinal direction of each of the diffusor blades 14 defines a flow direction of the liquid. The arrangement of the diffusor blades 14 is thus symmetrical in the axial direction.
The diffusor blades 14 define the flow direction of the treating liquid on the side of the diffusor 12. The existence of the diffusor blades 14 forces the liquid flowing on the side of the diffusor 12 to exhibit a flow straightening. As a result, the flow rate of the liquid is reduced. Concurrently, a rise of pressure of the liquid appears. Namely, the diffusor blades 14 make the flow rate of the liquid decrease and cause the pressure rise of the liquid.
The conventional diffusor pumps, and particularly the LNG immersed diffusor pumps are, however, burdened with the following disadvantages in the flow of the liquid along the side portion of the diffusor 12. Under a normal condition, such axially symmetrical diffusor blades 14 accomplish the above mentioned effects of both increasing the pressure of the liquid and its forced flow straightening. Thus, when a discharge flow rate of the liquid is within a reference discharge flow rate range, such axially symmetrical diffusor blades 14 are able to exhibit excellent functions of pressure increase and flow straightening of the liquid. Under the normal state in the reference discharge flow rate range, there exists no problem in the flow of the liquid. However, if the liquid has a discharge flow rate below in the reference discharge flow rate range, the flow of the liquid in the vicinity of the diffusor blades 14 exhibits a revolution and a stall. The undesirable phenomenon of the revolution and the stall of the flow of the treating liquid causes an axial vibration of the diffusor 12. This makes the life-time of bearings of the diffusor 12 short. This also makes the Q-H property inferior thereby lowering the pump efficiency. In addition, when the revolution and the stall of the flow of the liquid appear, the hunting of the flow of the treating liquid also appears. The above mentioned undesirable phenomena are considerable in the LNG immersed diffusor pumps.
To prevent the above undesirable phenomenon in the flow of the liquid, it is required that the discharge flow rate of the treating liquid is so controlled as to be within the reference discharge flow rate range. In the prior art, the operation of such LNG immersed diffusor pump is restricted, if the discharge flow rate of the treating liquid is lower than a discharge flow rate at which the revolution and the stall of the treating liquid flow appear. Under such lower discharge flow rate, the diffusor pump is not operative. It is desirable to lower as much as possible a critical discharge flow rate at which the revolution and the stall occur so that the operative range of the discharge flow rate becomes wide. It is, therefore, required to develop novel diffusor blades of the diffusor pump, which are able to make the diffusor pump operative in a wide range of the discharge flow rate of the liquid.